Adsorption of Phosphonic Acid at the TiO2 Anatase (101) and Rutile (110) Surfaces

The adsorption of phosphonic acid on the TiO2 anatase (101) and rutile (110) surfaces have been investigated by means of efficient density-functional-based tight-binding calculations. We studied the geometries and adsorption energies of several adsorption models to achieve clarification of the discrepancy in the experimental finding of a preferred binding state. In this paper we show that there are several adsorption structures likely to be present on the specific TiO2 surfaces. Those structures have exclusively a bidentate configuration. They have similar adsorption energies but different geometries. For the monodentate complexes, we find a strong trend of the adsorption geometry relaxing toward the bidentate coordination. Also, they have significantly smaller adsorption energies. Furthermore, we extensively demonstrate the reliability of the SCC-DFTB method for this chemical system, which opens the way for studies of adsorption on more complex titania materials.